Method of making electrical coils



Nov. 14, 1967 F. s. NICHOLS ET AL METHOD OF MAKING ELECTRICAL COILSOriginal Filed Dec.

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United States Patent O 3,352,711 METHOD @F MAKING ELECTRICAL CILS FrankS. Nichols and Eugene K. Steele, Pittsfield, Mass., assignors toIGeneral Electric Company, a corporation of New York @riginalapplication Dec. 1, 1959, Ser. No. 856,434, now Patent No. 3,138,773,dated .lune 23, 1964. Divided and this application Feb. 17, 1964, Ser.No. 345,217

8 Claims. (Cl. 117-201) ABSTRACT OF THE DISCLOSURE A method of treatingelectrical coils which comprises applying to the coil a liquidsuspension of fusible synthetic resin particles and thereafter fusingthe resin particles deposited on the surfaces of the coil.

This application is a division of co-pending application Ser. No.856,434, led December l, 1959, and assigned to the same assignee as thepresent application, now Patent No. 3,138,773, dated lune 23, 1964.

The present invention relates -to electrical coils and particularly toinduction coils for electromagnetic apparatus such as transformer andthe like.

In the manufacture of coils of the above type, it has been the practiceto apply an insulating coating of varnish on the coil. By this means,the mechanical strength of the `wound structure is increased foraifording better resistance to the stresses involved in the coilhandlingand assembly procedures, and especially to the high mechanicalVforces to which the coils may be subjected during operation of theelectrical apparatus. For example, under short circuit conditions, highmechanical stresses tend to separate the core windings both axially andradially and may lead to distortion or even rupture of the coilstructure. In the past, varnish compositions employed for coating thecoils have included alkyd resin, phenolic resin, and polyester resinvarnishes, the resin material being dissolved in a suitable volatilesolvent. However, it has been found that such varnish-treated coils arenot fully satisfactory for use in oil-filled apparatus where thedielectric oil or other liquid is intended to impregnate the coil toprovide adequate resistance to high electrical imp-ulse conditions. Itappears that the previous varnish treatments resulted in aliquid-impermeable coating being formed over the coil structure whichelectively prevented the ready penetration of the dielectric liquid intothe many small crevices and interstices inevitably present in the woundcoil structure. As a result, there was a greater probability of failuredue to impulse conditions than where Asuch varnish treatment wasomitted. On the other hand, elimination of such varnish treatment couldbe made only at the sacrifice of short circuit strength of the coil.

It is an object of the invention to provide an electrical -coilstructure which has satisfactory mechanical strength to resist shortcircuit conditions as well as being capable of resisting high electricalimpulse conditions.

It is a specic object of the invention to provide an electrical coil ofthe above type having an insulating coating thereon which affordssatisfactory mechanical bonding for the coil turns while permitting thedielectric liquid in which the coil is immersed to penetrate into theinterstices in the coil.

It is a further object of the invention to provide a method of treatingelectrical coils to impart the abovementioned improvements thereto.

Other objects and advantages will become apparent from the followingdescription and appended claims.

Wi-th the above objects in View, the present invention relates to anelectrical apparatus comprising, in combination, a container having adielectric fluid therein, a coil in the container immersed in thedielectric fluid therein, the coil being formed by a wound conductorhaving an insulating coating thereon, the coil having intersticestherein and having deposited thereon a film formed of a fusedparticulate resin material, the film being deposited on the coil from asuspension of polymer particles in a nonsolvent liquid medium, wherebythe deposited iilm firmly bonds the wound coil while permittingpenetration of the dielectric fluid into the interstices 0f the coilwinding.

The invention Will be better understood from the following descriptionand accompanying drawing in which:

FIG. 1 is a perspective view partly in section of an electricalinduction coil embodying the present invention; and,

FIG. 2 shows a transformer partly in section in which the FIG. 1 coil isincorporated.

Referring now to the drawing and particularly to FIG. 1 there is shown atypical transformer coil structure A comprising an inner insulatingcylinder 1 of pressboard or the like on which the coil turns are wound,the coils including a low voltage winding 2 and a high voltage Winding 3which are radially separated from each other by duct forming spacers 4such as wooden strips or the like. Each coil winding consists Iof aplurality of radially superposed layers of helically Wound wire 5, thewire being coated with insulating enamel such as a phenolicmodifiedpolyvinyl formal, and the layers of wire being separated from each otherby cellulosic paper insulation sheets 6. In accordance with theinvention, the surfaces of the coil structure are covered with a -iilmof resin 7 deposited from a suspension of polymeric particles in anon-solvent medium such as water.

FIG. 2 shows the coil structure of FIG. 1 operatively assembled in atransformer which comprises a tank 8 closed by a cover 9 and containinga dielectric liquid 10 such as mineral oil or the like. In thetransformer, the coil structure -A encircles magnetic core 11, and thehigh and low voltage leads 12, 13 of the coil are respectively connectedto corresponding bushings 14, 15 mounted on cover 9.

By virtue of the particular type of resin coating provided on coil A, ithas been unexpectedly found that effective permeation of the dielectricliquid 10 into the inner structure and interstices of the coil unit isnot prevented and that as a result a substantial decrease in the numberof failures on impulse testing of such coils is achieved. This benefitis obtained while still retaining a good bond between the coil turns bymeans of the resin coating, thus providing for adequate short circuitstrength of the coil.

The material which is applied to the coil to obtain the desired resincoating may be of known types of compositions consisting essentially ofsuspensions of extremely small particles of solid polymers in water, theparticle size being typically about l to 500 microns. These materialsare normally made by the emulsion polymerization of one or more liquidpolymerizable monomers, the emulsication of the original monomersproducing the very small size particles of polymer. The monomers usedmay be difunctional giving rise to thermoplastic polymers, commonexamples used commercially being vinyl acetate and various acrylicesters. Alternatively, the monomers may be polyfunctional provided thattwo of the functional sites may be preferentially reacted to produceminute particles of a thermoplastic polymer which by further reactioncan convert to a thermoset polymer. Materials of this latter typeprepared, for instance, from acrylonitrile, butylacrylate, methacrylicacid, and glycidyl methacrylate are described in Sanders Patent 2,787,-

3 561. A specific example of the latter composition is as follows:

EXAMPLE I Ingredients: Parts by wt.

Polymerizable monomers:

Acrylonitrile 63.0

Butyl acrylate 32.0

Methacrylic acid 3.0

Glycidyl methacrylate 2.0 Polymerization initiator:`

Sodium bisultite 0.1

Potassium persulfate 0.3y Dispersing agent:

Sodium lauryl sulfate 0.5 Telogen:

Dodecyl mercaptan 0.5 Water 200.0

Additional suitable compositions of thermosetting types are disclosed inthe patent to Erickson et al. 2,580,901, of which the following areillustrative:

EXAMPLE II Parts Styrene 180 Glycidyl methacrylate 20 Dioctyl sulfosuccinate 5 Water 580 Ammonium persulfate 0.1

EXAMPLE III Ethyl acrylate 120 Glycidyl methacrylate 80 Sodium laurylsulfate 1.5 Water 300 Ammonium persulfate 0.1

Other types of compositions of thermoplastic nature which may be used inpracticing the invention are:

EXAMPLE IV Parts Water 512 Vinyl acetate 276 Ethyl acrylate 161 Butylacrylate 23 Sodium lauryl sulfate 7.5 Ammonium persulfate 0.5 Sodiumbicarbonate 0.4

EXAMPLE V Ethyl acrylate 29.26` Methyl methacrylate 14.41 Methacrylicacid 0.67 Sodium lauryl sulfate 0.30 Sodium bicarbonate 0.05 Ammoniumpersulfate (4% solution) 2.42 Sodium methabisultite solution) 1.00

Water 40.34 Ammonium hydroxide sufiicient to bring pH of productsuspension to 8.5 to 9.0.`

In all cases, the product utilized in practicing the invention is astable suspension of minute fusible particles of polymer in anon-solvent medium, preferably water. In use, this suspension is coatedonto the coil and as it loses water by drying, the suspension becomesunstable and the polymer particles are deposited as a coating. Theparticles may subsequently be baked to fuse them together and, in thecase of the thermosetting types, to complete the sec- 0nd stage ofpolymerization and cure them. If desired, small amounts of solvent forthe polymer may be added to the suspension to facilitate fusion of thepolymer particles without adversely affecting the operation of thematerial for the purpose of the invention. We have found that if anelectrical coil, particularly one insulated with cellulose material, isdipped in such a material and dried (or cured where appropriate), whileLwater from the suspension may penetrate into the coil, the solidparticles are deposited on the surface only and that while such acoating provides excellent mechanical reinforcement, it surprisinglydoes not prevent subsequent penetration of oil into the coil.`

Several hundred distribution transformer coils have been treated with acommercially available acrylic polymer suspension of the above-describedtype, and it -has been demonstrated that this treatment substantiallyimproved short circuit strength While satisfactory impulse tests onthese coils showed that complete penetration of the dielectric liquidtook place. In contrast, treatment of the coils with conventional alkydvarnish has been found in tests to result in six times as many failuresof the coils in impulse testing.

Another series of tests Was conducted to determine the yeffectiveness ofthe described polymer suspension material in providing adequate bondingof the coil turns to resist hig-h electrical and mechanical stresses. Inthese tests, in which the tested coils were subjected to short circuitconditions, it was found that coils having insulation pretreated withshellac in accordance with a commonly employed procedure withstood only37 times the normal operating current before showing damage due tophysical shifting of the turns, whereas similar coils which had beenadditionally treated by dipping them into an` aqueous acrylic polymersuspension of the above-described type withstood 50 times the normaloperating current before such damage resulted. This test thusdemonstrated the excellent coil-bonding properties of the polymersuspension coating material.

In a typical process for treating transformer coils in accordance withthe invention, the coil is dipped at room temperature in a suspension ofpolymer material having the composition set forth in Example I above,the coil being typically formed of wire covered with a polyvinyl formalenamel and wound on a porous laminated paper or pressboard winding form.The viscosity of the suspension bath may be adjusted as desired byadding water. Thedipping period is not critical and may vary from a fewseconds tofseveral minutes. It has been found that due to thenature ofthe polymeric suspension even a single shortdipping period is effectiveto provide adequate bonding of the coil turns to provide high resistanceto shortcircuit conditions, in contrast to conventional varnishes whichnormally require a relatively long or repeated dipping to provideequivalent mechanical strength. After the coil is dipped, it is removedfrom the bath and allowed to drain for a short period. Thereafter, thecoil is allowed either to air dry and then curing is effectedrduring theusual transformer drying cycle, or, the thus treated transformer coilmay be baked initially at a temperature below the boiling point of waterand when most of the water has been driven off, the temperature israisedto about C. for a sufficient time to cure the polymer.

The bake cycle, as indicated above, is used either to fuse the polymericparticles (as in the case with suspensions of thermoplastic materials,e.g., polyvinyl acetate) or to fuse and cross-link the polymer chain (asin the case of thermosetting materials such as in the abovementionedSanders patent). Thereafter, the dried core and coil assembly is placedin a transformer tank and the tank is filled with appropriate`insulating liquid.

While it appears that improved results are obtained by the presentinvention because the dielectric liquid of the electrical apparatuspermeates the coated coil more readi- 1y` than in the case of priortypes of coated coils, the reason for the improved permeability is notfully understood. It may be that the particulate resin material,although fused to a practically continuous film, leaves unbridged gapsespecially at the ends of the coil between layers, through which thedielectric oil may pass, whereas prior solution-types of resin coatingshave formed an impervious bridge across such irregular areas orfissures` and the dielectric liquid is thus eifectively prevented fromentering the interior of the coil to saturate the intersticesy therein`While the present invention has been described with reference toparticular embodiments thereof, it will be understood that numerousmodifications may be made by those skilled in the art without actuallydeparting from the scope of the invention. Therefore, the appendedclaims are intended to cover all such equivalent variations as comewithin the true spirit and scope 0f the invention.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. The method of treating electrical coils for improving theirresistance to electrical impulse conditions while providing for highresistance to short circuit conditions which comprises applying to thecoil a liquid suspension of fusible synthetic resin particles andthereafter fusing the resin particles deposited on the surfaces of thecoil.

2. The method of treating electrical coils for improving theirresistance to electrical impulse conditions while providing for highresistance to short circuit conditions which comprises applying to thecoil a suspension of fusible resin polymer particles in a liquidnon-solvent medium, and curing and fusing the polymer particlesdeposited on the coil while removing the non-solvent medium, whereby aresin coating is provided on the coil which firmly bonds the turnsthereof while permitting permeation of dielectric liquid therein.

3. The method of treating electrical coils which comprises coating thecoil with a suspension of fusible resin polymer particles in a liquidnon-solvent medium, and thereafter heating the thus applied coating tocure and fuse the polymer particles deposited on the coil while removingthe liquid non-solvent medium, whereby a resin coating is provided onthe coil which firmly bonds the turns thereof while permittingpermeation of dielectric liquid therein.

4. The method of treating electrical coils which comprises coating thecoil with a suspension of fusible resin particles composed of athermosetting acrylic polymer in a liquid non-solvent medium, andthereafter heating the thus applied coating to cure and fuse the polymerparticles deposited on the coil while removing the liquid non-solventmedium, whereby a resin coating is provided on the coil which firmlybonds the turns thereof while permitting permeation of dielectric liquidtherein.

5. The method of treating electrical coils which comprises coating thecoil with a suspension of fusible resin particles composed of athermosetting acrylic polymer in water, and thereafter heating the thusapplied coating to cure and fuse the polymer particles deposited on thecoil while removing the water, whereby a resin coating is provided onthe coil which firmly bonds the turns thereof while permittingpermeation of dielectric liquid there- 1n.

6. The method of treating electrical coils which comprises coating thecoil with a suspension of fusible resin polymer particles in a liquidnon-solvent medium, drying the thus applied coating to remove thenon-solvent medium, and heating the thus treated coating to cure andfuse the polymer particles deposited on the coil, whereby a resincoating is provided on the coil which firmly bonds the turns thereofwhile permiting permeation of dielectric liquid therein.

7. The method of treating electrical coils which comprises coating thecoil with a suspension of fusible resin polymer particles in Water,heating the thus applied coating at a temperature below the boilingpoint of water to remove most -of the water therein, and thereafterheating the thus treated coating at a higher temperature for asufiicient time to cure and fuse the polymer particles deposited on thecoil, whereby a resin coating is provided on the coil which firmly=bonds the turns thereof while permiting permeation of dielectric liquidtherein.

8. The ymethod of treating electrical coils which comprises dipping thecoil into a suspension of fusible resin polymer particles in a liquidnon-solvent medium, removing the coil from the suspension, andthereafter heating the coating th'us applied on the coil to cure andf-use the polymer particles deposited on the coil while removing thenon-solvent medium, whereby a resin coatin'g is provided on the coilwhich firmly bonds the turns thereof while permiting permeation ofdielectric liquid therein.

References Cited UNITED STATES PATENTS 1,661,844 3/1928 Miller 117-232 X2,319,852 5/ 1943 Doolitt-le 117-201 X 2,686,738 8/1954 Teeters 117-232X 2,938,153 5/ 1960 Netherwood 117-21 X 2,942,217 6/1960 Ford 336-205FOREIGN PATENTS 529,881 9/ 1956 Canada. 342,291I 12/ 1959 Switzerland.

WILLIAM L. JARVIS, Primary Examiner.

1. THE METHOD OF TREATING ELECTRICAL COILS FOR IMPROVING THEIRRESISTANCE TO ELECTRICAL IMPULSE CONDITIONS WHILE PROVIDING FOR HIGHRESISTANCE TO SHORT CIRCUIT CONDITIONS WHICH COMPRISES APPLYING TO THECOIL A LIQUID SUSPENSION OF FUSIBLE SYNTHETIC RESIN PARTICLES ANDTHEREAFTER FUSING THE RESIN PARTICLES DEPOSITED ON THE SURFACES OF THECOIL.